Flatten object without distorting it?
You can either scale the whole sword in object mode by selecting it and pressing S > X to scale along the X axis or,

If you prefer a little more detail select your sword, go into edit mode with Tab key, select only the faces corresponding to the blade part, and scale them along the X axis with S > X.

If you go with the first option of scaling in object mode don't forget to apply the scale afterwards by pressing Ctrl + A > Scale when you're done.

P.S. Scaling (and other modal operators in Blende) use the cursor distance to the center for the screen at the moment of invoking the operator for precision factor, so make sure your mouse is not too close to the center when calling the command or you may end up with an overly sensitive scaling factor that may yield weird results as in your second image.